Olefins from cat-cracked cycle oil



Uflitti l OLEFINS FROM CAT-CRACKED 'CYCLE OIL No Drawing. Application October 20, 1955 Serial No. 541,826

11 Claims. (Cl. 260-460) This invention relates to a process for producing olefins having at least carbon atoms or mixtures of such olefins in pure or practically pure state. The invention further relates to the production of derivatives from mixtures of these olefins, in particular arylalkanes suitable for conversion to surface-active alkylaryl sulfonates, and to the alkylaryl sulfonates so produced.

Heretofore, it has been known to produce mixtures of olefins of 5 or more carbon atoms per molecule, substantially free of other hydrocarbon types, by the vapor phase cracking of para'lfin wax and fractionation of the liquid cracked product obtained. Olefins so produced, having from 8 to 18 carbon atoms per molecule, may be suitable for the production of detergent alkylate, i. e., arylalkanes produced by the alkylation of benzene or other aromatic hydrocarbons with these olefins and adapted to be converted into detergents by sulfonation and neutralization. These C to C olefin fractions from wax cracking have high contents of alpha olefins, typically in excess of 90% by Weight of the total fraction, and have a very low content of cyclo-olefins with the unsaturation in the ring, e. g., 1 to 2% of the total, and a low content of cyclic compounds with olefin chains attached, e. g., 7% of a C to C fraction.

Olefins suitable for the production of detergent alkylate have also been produced from certain other sources, e. g. by the polymerization of propylene. Detergent alkylate of quality to produce a satisfactory detergent upon sulfonation has only been produced by careful selection of particular olefin feed stocks. It is generally thought that olefins suitable for production of detergent alkylate should have a straight or slightly branched chain, generally having in the range from 8 to 18 carbon atoms per molecule.

The process according to the invention consists in catalytically cracking a petroleum oil including components boiling above 300 C., fractionating the cracked products, recovering a fraction boiling above 300 C., selectively removing aromatic compounds from the fraction, e. g., by extraction witha selective solvent, thermally cracking in vapor phase the resulting substantially aromatics-free raifinate and recovering from the thermally cracked products olefins having from 5 to 18 carbon atoms per molecule. According to the present invention, fractions of olefins having in the range between 8 and 18 carbon atoms per molecule, produced in this manner, may then be employed to alkylate benzene or other suitable aromatic hydrocarbons in the presence of an alkylation catalyst, e. g. hydrogen fluoride, to produce detergent alkylate which is recovered, sulfonated with sulfuric acid and neutralized to produce a detergent. In another process according to the present invention, a fraction having from 8 to 18 carbon atoms per molecule recovered from the vapor phase cracking operation may be reacted with concentrated sulfuric acid and the sulfation product converted to sodium alkyl sulfates suitable as detergents. Olefins produced according to the present process may also be converted into alkyl salicylates or alkyl aromatic sulfonates suitable as lubricant additives.

States Patent 0 "ice In the catalytic cracking of petroleum oils containing heavy fractions boiling above 300 C., e. g., heavy distillates, reduced crude and the like, the reactions which take place include not only the cracking of paraffinic and of naphthenic compounds but also include isomerization to form more highly branched parafiinic compounds, formation of polycyclic aromatics and other reactions. As a result, the cycle oil recovered from the process, i. e. the distillate fraction boiling above 300 C., contains not only compounds originally present in the charge to the process but also compounds resulting from the conversion. This oil will contain polycyclic aromatic compounds, naphthenic compounds, branched paraflins and normal paraflins. The composition of this product differs greatly from the straight run paraflinic waxes which have previously been used for the production of olefins by vapor phase cracking. The latter comprise substantial proportions of normal and very slightly branched paraffins and very few cyclic compounds. Contrary to expectations, therefore, it was found that the heavy cycle oil from-whichthe aromatic compounds have been removed by an extraction step is an excellent initial material for the production of olefins suitable for conversion into detergent alkylate and other chemical products in yields that are not lower and in some cases even higher than those obtained by the use of an initial material such as parafiin wax.

Olefin derivatives, such as higher alcohols, sulfonic acids, sulfuric acid alkyl esters or their salts, and the like, may be prepared in known manner from the olefin mixtures obtained by means of the process according to the invention. Thus, for example, the C -C olefin fraction or a higher olefin fraction .may be processed by Oxo synthesis to higher alcohols which are specially suitable for the preparation of esters which are valuable, inter alia, as plasticizers for polyvinyl chloride, while the fractions containing olefins with 9 and more C-atoms are specially suitable for the preparation of surfaceactive derivatives containing a sulfonic acid group or a sulfuric acid ester group. The olefins obtained according to the invention, especially those containing at least 8 carbon atoms, have been found particularly suitable for conversion into arylalkanes by means of aromatics, such as benzene, toluene, naphthalene, and the like, in the presence of an alkylation catalyst such as HF, and from these arylalkanes alkylaryl sulfonates with very good surface-active properties can be prepared by sulfonation.

For this purpose relatively narrow olefin fractions having a carbon number spread of no more than four, i. e. olefins, in which the number of carbon atoms per molecule does not vary by more than 4, such as a (E -C fraction, when it is desired to prepare a liquid final product, and a C -C or G -C fraction if it is desired to prepare a powdery final product, are specially suitable. By alkylation of benzene with such fractions, with the use of HF as catalyst, alkylates can be obtained which when processed to alkylaryl sulfonates yield products which have excellent properties, particularly with regard to detergency. Separate alkylates may also be prepared from various fractions, such as a C C and a C C olefin fraction, and afterwards they may be mixed, the alkylate mixture obtained being processed to alkylaryl sulfonates.

The olefins or olefin mixtures obtained according to the invention, such as a C C fraction, may also be advantageously converted in known manner into surfaceactive secondary sulfuric acid alkyl ester salts, such as C -C sodium alkyl sulfates, or into alkylsalicylates, such as calcium alkylsalicylates, or calcium salts of alkylaromatic sulfonic acids which constitute valuable lubricant additives.

The present invention is of particularly great practical importance because it enables the heavy cycle oil, which is produced in great quantities in catalytic cracking processes,'and which, if not recycled for economic reasons, constitutes a by-product of relatively little value, to be used for the preparation of valuable higher olefins and their derivatives for which there is a constantly increasing need. Heavy cycle oil obtained in the catalytic cracking of mineral oils rich in sulfur, such as certain Middle East oils, has a relatively high sulfur content, 'ye. g. 2 to 3%. In extracting a heavy cycle oil of this type, the sulfur compounds present are largely removed together with the aromatics, so that a practically sulfurfree ratfinate is obtained.

In the catalytic cracking step of the present invention, the catalyst may be a natural or synthetic composite of silica and alumina which may be employed as a fiuidized solid, as a moving bed or a fixed bed in a manner well known to the art. The feed to the catalytic cracking operation is a petroleum oil containing a substantial proportion, e. g. between 50 and 100%, boiling above 300 C. This may be a heavy distillate, e. g. boiling from 300 to 600 C., or it may be a topped or a reduced crude oil. Temperatures in the cracking step generally are in the range between 430 and 540 C. at relatively low pressures, e. g., between and 15 lbs. per sq. inch. The catalyst employed in the catalytic cracking step is periodically regenerated by burning with a free oxygencontaining gas in conventional manner. The cracking operation may be a so-called once through operation in which only fresh feed is charged to the cracking unit or it may be a recycle operation in which a fraction of the efiluent, boiling above 300 C., is returned to the catalytic cracking unit. Superior feed stocks for the thermal cracking step of the present invention are produced by operating the catalytic cracking step at relatively severe conditions, i. e., at temperatures above 485 C. when operating once through, or at recycle rates (ratio of total feed to fresh feed) of between 1.2 and 2.0. Superior feed stocks for the thermal cracking step of the present invention are also produced when the oil charged to the catalytic cracking step is produced from a highly paraflinic crude, e. g., a Middle Eastern crude, such as Kuwait, or a Pennsylvania crude.

The effluent from the catalytic cracking step is fractionated into several fractions including the lighter hydrocarbon fractions, such as the normally gaseous hydrocarbons, gasoline, kerosene and others, and including a distillate fraction boiling above 300 C. and a bottoms fraction. The distillate fraction boiling above 300 C. is generally referred to as cycle oil in the fluidized catalytic cracking process because frequently the whole or a part of this fraction is recycled back to the cracking step. The bottoms fraction obtained in the fluidized operation is referred to as slurry oil because it generally contains some of the fluidized catalyst carried over into the product fractionator. This catalyst may or may not be separated, and the slurry oil also is often returned to the cracking step.

According to the present invention, the cycle oil, either by itself or admixed with the slurry oil, is subjected to an extraction treatment to separate the aromatics therefrom. The extraction may be carried out by liquid-liquid contacting with a selective solvent for aromatics, or by contact with a solid adsorbent selective for aromatics. A variety of known solvents may suitably be employed to secure the desired selective removal of aromatics, e. g., furfural, liquid sulfur dioxide, mixtures of phenol and water, and the like. The treatment is preferably carried out by countencurrent contact in at least three extraction stages. Furfural and mixtures of phenol and water were found particularly suitable for purposes of the present invention, since a practically aromatics-free raffinate was easily obtained therewith even when using moderate solvent ratios, e. g., between 2.0 and 3.5.

Liquid sulfur dioxide was also found to be suitable. The extraction treatment is conveniently carried out at a slightly elevated temperature, e. g., between 50 and C. Solid adsorption separation may be carried on by use of silica gel as adsorbent.

The extraction treatment is preferably carried out by counter-current contact. Although a variety of equipment may be employed, an extraction column such as described in U. S. Patent No. 2,601,674 to G. H. Reman may be advantageously used for this purpose, the column being provided with a rotatable shaft with a number of discs and, on its inner wall, at points approximately half way between the discs, with rings the internal diameter of which is larger than the diameter of the discs.

Extraction conditions are selected to produce a raffinate containing no more than 5% by weight and preferably no more than 1.5% by weight of aromatics.

The thermal cracking step of the present invention is carried out at conditions which are conventional for the vapor phase cracking of Wax to produce higher olefins, e. g., by heating the vapors of the rafiinate together with an inert diluent, conveniently steam, to a high temperature in the range from 500 C. to 600 C., preferably between 550 and 560 C., for a short period, c. g. up to a few seconds, at pressures between 1 and 5 atmospheres. The cracked product obtained, after separation of the normally gaseous components, can then be separated by fractional distillation into the desired higher olefin fractions boiling up to 350 C. and a high boiling bottom product which may be recycled to the thermal cracking zone.

The olefins produced according to the present process generally boil in a range up to 350 C. It is therefore essential that the charge to the vapor phase thermal cracking operation contain no components boiling below 350 C. because these would contaminate the substantially completely olefinic product. In one modification of the process, therefore, the cycle oil charged to the extraction step is distilled to remove components boiling below 350 C. It has been found, however, that components boiling between 300 and 350 C. are substantially entirely aromatic in nature and are removed with the extract, so that even when charging a cycle oil of an initial boiling point of 300 C., the rafiinate resulting from the extraction step had the desired initial boiling point above 350 C. The C -C product produced in the vapor phase cracking step has an olefin concentration between and by Weight or higher.

The aromatic extract which is obtained in the extraction of the heavy cycle oil and chiefly consists of polycyclic compounds, such as naphthalene derivatives, may be employed as fuel, as a heat transmission agent, as a component in weed killers or as a source of chemical intermediates.

The detergent alkylate may be prepared by reaction of the olefins produced as stated with a suitable aromatic 1 in the presence of an alkylation catalyst. The preferred aromatic is benzene, although toluene and other monocyclic alkylaromatics may be employed. It is generally desirable to maintain in the reaction mixture a molecular ratio of aromatic hydrocarbon to olefin of at least 5:1, preferably about 10:1, and up to :1. Suitable alkylation catalysts are sulfuric acid, aluminum chloride-hydrocarbon complexes, and substantially anhydrous hydrogen fluoride. When liquid catalyst, such as concentrated sulfuric or hydrofluoric acid is employed, it is preferably used in approximately equal volume with the hydrocarbon reactants. Thus, a catalyst liquid-to-hydrocarbon volume ratio of about 1:1 is preferred, but lower or greater ratios may be employed. The preferred catalyst in the present process is liquid hydrogen fluoride. Its concentration may range from 85% to 100% HP, its water content being maintained very low, e. g. no higher than 1 or 2% by weight, the remainder being dissolved hydrocarbon material.

Suitable temperatures vary, 'cle- 5 pending on the catalyst, but are generally in the range between and 50 C.

The alkylated aromatic fraction is recovered from the alkylation reaction mass and is sulfonated in known more than four. Suitable ratios of olefin to. phenol are, for example, 1.1:1 and 2.2:1. Higher selectivity is obtained at the former ratio. Suitable alkylation catalysts are ZnCl HCl and ZnOHC1. Alkylation is prefermanner, e. g. by contact with an excess of concentrated 5 ably carried out at temperatures of about 140l50 C. sulfuric acid. The sulfonation may be carried out at The alkylated phenol is recovered and is converted into temperatures up to 50 C. The acid concentration is sodium phenate by reaction with metallic sodium or with preferably at least 97%. Acid up to 100% concentra- NaOH in an organic diluent, e. g. methanol, ethanol, tion and oleum with up to 20% S0 or higher may be butanol, methyl-isopropyl ketone, and the like. The employed. With higher acid concentration, lower reacsodium phenate may be converted by the Kolbe synthesis tion times are required, e. g. about 8 hours with 98% to the carboxylate by reaction with CO e. g. at 130- acid and one hour with 100% acid. Volume ratios of 140 C. and 400 p. s. i. g. pressure for 4 hours. The sulfuric acid to hydrocarbon may range from 0.8:1 to product contains sodium alkyl salicylate and sodium 1.25:1, a 1:1 ratio is suitable. The larger the ratio, the phenate. The product is acidified to give alkylphenol more inorganic sulfate will be present in the product, and the desired alkylsalicylic acid. The latter is neufollowing neutralization. In many cases, the inorganic tralized with compounds of the desired metal, e. g. calsulfate is a desirable constituent of the finished detergent cium or zinc. When employing neutral salts, e. g. CaC1 composition. or ZnCl the neutral salicylate is produced. When em- The sulfonation product mixture is preferably freed, ploying bases, e. g. Ca(OH) CaO or ZnO, the basic e. g. by decanting, from unsulfonated hydrocarbons. The alkylsalicylate may be recovered. mixture is then neutralized, the sulfonic acids being thus The process according to the invention will be further converted to sulfonic acid salts and the excess sulfuric illustrated by means of the following examples: acid into sulfate. The neutralization may be carried out with any base or basic-reacting inorganic or organic sub- EXAMPLE I stance. Thus, to produce sodium sulfonates, aqueous A sample of combined heavy cycle oil and slurry oil, sodium hydroxide or sodium carbonate are suitably emboiling above 300 C., was obtained from a commercial ployed. Other alkali metal, alkaline earth metal, amfluid catalytic cracking unit operating on a Middle East monium or amine salts may be similarly produced from paraflinic heavy distillate charge at a total feed to fresh the corresponding basic compounds. The neutralization feed ratio of 1.25, a conversion of 51.8% by weight, a is generally carried out by contact with the aqueous solureaction temperature of 480 C. and a catalyst to oil tion at temperatures from 20 to 100 C., those between ratio of 4.8, with a silica-alumina cracking catalyst of 80 and 100 C. being preferred. 82 m. g. surface area. The fraction boiling above 350 Alkyl sulfates may be prepared by reaction of the ole- C. comprised 85.5% by weight of the total cycle oil plus fins produced as stated with concentrated sulfuric acid, slurry oil. An analysis of this fraction for isoparafiins e. g. 96%, under conditions at which the olefins are plus naphthenes, normal paraflins plus monomethyl absorbed by the acid while undesired side reactions, such branched isoparafiin, total parafiins, and aromatics is as tar formation, are avoided. The acid may be slowly shown in Table I. A similar analysis is shown for 10% added while the mixture is cooled and agitated to keep fractions of the portion of the same oil which boiled the temperature low, e. g. between 5 and 10 C. The above 350 C. It is seen that over the range from 20 absorption product is hydrolyzed, e. g. by admixing with to 90% of this fraction, normaland monomethyl ice, and an alkyl sulfate layer separated from an aqueous parafiins comprise between 20 and 30%, isoparaflins acid layer. The alkyl sulfate is then converted to the and naphthenes between 30 and and aromatics the corresponding metal salt by reaction with an aqueous remainder.

' Table 1 Product Saturates 'Aro- Sulfur, Fraction Isoparaflins n-paraffins and matics, percent Cycle Oil Slurry Oil 350 O. and monomethyl Total, percent wt.

naphthenes, branched percent wt. Fractlon 350 0. percent wt. isoparaifines, wt.

percent wt.

Percent wt. on feed 85. 5 Percent wt. on 350 C 100. 0 85. 5 20. 5 56 44 Percent wt. of 350 0.:

0-1 14. e 8. 8 23. 4 76. a 10-20 27. 1 10. a 43. 4 50. 0 20-30 33. 0 21. 0 54. 0 4s. 0 30-40 43.4 24.8 68.2 31.8 40-50 48. 6 2s. 8 77. s 22. 7 50-00 48.1 23.1 71.2 28.8 60-70 44. 1 20. 1 70. 2 29.8 70.30 41. 6 24. 4 66. 0 34. 0 -90 37. 3 22. 0 59. 3 40. 7 -100 18. 0 10. 8 2s. 8 71. 2 Feed to Solvent Extractiom- 46. 4 53. 6 Extract, percent w 11.0 89. 0 Raflinate, percent wt 63. 5 35 98. 5 1. 5

base of the desired metal, e. g. sodium hydroxide or the hydroxide of another alkali metal, of an alkaline earth metal, or of ammonia.

Olefins prepared as stated are useful in the production of alkylsalicylates which, in the form of certain of their metal salts, are employed as additives in lubricating oils. These additives are prepared, e. g., by alkylating phenol or a phenolic compound with a fraction of'said olefins having between 8 and 18 carbon atoms per molecule and preferably having a carbon number spread of no 75 Equivalent theoretical stages Feed l./h 13.5 Solvent ratio, vol. furfural/vol. feed 3.5 Rotor speed R. P. M 300 Temperature of contactor C.. 60-70 Temperature of top settler C 60-70 Temperature of bottom settler C 55-65 The yield of ratfinate. was 41.7% by weight of the oil charged. The concentration of saturated compounds, aromatics and sulfur in the extract and rafiinate is shown in Table I. It is seen that the rafiinate contained 63.5% of lsoparafiins and naphthenes and 35% of normal parafiins and monomethyl branched isoparafiins and only 1.5% of aromatics. The sulfur content of the charge oil was substantially entirely removed in the extract. The charge to the solvent extraction had an initial boiling point of 246 C. and a boiling point of 348 C. The rafiinate had an initial boiling point of 356 C. Thus the rafiinate had a boiling point suitable for production of olefins boiling up to 350 C. without requiring prior topping of the cycle oil. The raflinate fro-m thefurfural extraction was cracked in vapor phase. It was first vaporized as completely as possible ata maximum temperature of about 470 C., together with the fraction of the vapor phase cracked product boiling above 300 C. which was recycled in a ratio by weight of 2.9:1 with reference to the rafiinate. The vapors, after separation of the unvaporized material present, were passed through a cracking furnace at a maximum temperature of about 550 C. for a contact period of 3.6 seconds, together with 6.5% by weight of steam. The portion of the cracked product boiling above 300 C. was recycled as already stated, while the portion boiling below 300 C. was separated by fractional distillation into a fraction containing normally gaseous olefins, a fraction containing pentenes, a C C olefin fraction, a C C olefin fraction, and a C -C olefin fraction. These fractions were obtained in amounts of 24.8%, 4.3%, 14.4%, 22.8%, and 22.8%, respectively of the weight of the charge raflinate. The olefin content of these fractions was 8590% by weight. A more detailed olefin analysis of the C C fraction and the C -C fraction obtained in this recycle operation is given in Table II, as well as a corresponding analysis of the product from cracking in the same manner a straight runpetroleum wax such as is ordinarily employed in vapor phase EXAMPLE II A C C -cracked fraction (boiling from 221 C. to

283 C.) was separated by fractional distillation from a cracked product obtained in the manner described in Example I, in a yield of 19.8% by weight based on the Weight of rafiinate charged in the cracking process.

This C -C -cracked fraction, of which the olefin content was 88% by weight, was coupled by continuous alkylation at 30 C. to benzene by means of liquid HF as catalyst in a reactor which was provided with a stirrer revolving at 220 R. P. M. The mol ratio of benzene to olefins was lOzl and the ratio by weight of the acid phase to the hydrocarbon phase 1:1; 875 parts by weight of cracked fraction benzene were charged per hour, and the residence time was about 30 minutes. After separation of the acid phase, which was recycled, the reaction mixture discharged from the reactor was subjected to fractional distillation at atmospheric pressure, 603 parts by weight of benzene being recovered, and in addition 226.5 parts by weight of purified alkylate with a boiling range (converted to 760 mm. Hg) of 285 C. to 390 C., a specific gravity 1 of 0.871, a refractive index ri of 1.489, and an average mol weight of 271. The alkylate yield, calculated on olefins, was 93.0 mol percent.

The alkylate thus obtained Was sulfonated with oleum of 20% in the conventional way, the yield being 97.7 mol percent, calculated on alkylate. After neutralization with aqueous sodium hydroxide, the sulfonation product yielded an alkylaryl sulfonate which, when used as a detergent in the form of a mixture with a high content of so-called builders, was found to have properties which were b tter than those of an alkylaryl sulfonate obtained by starting from an alkylatc of benzene with propene tetramer.

EXAMPLE III A C C -cracked fraction (boiling from C. to 300 C.) was separated from a cracked product, obtained according to Example I, in a quantity of 45% by weight based on the raffinate charged in the cracking process. This C C -fraction, of which the olefin content was 94% by weight, was sulfated with sulfuric acid of 96% at 15 C., for a reaction period of 20 minutes, the mol ratio of H 50 to cracked fraction being 0.8: 1.0, and the sulfation product processed to sodium alkyl sulfates in the usual way. The sodium alkyl sulfate yield was 39 mol percent based on the cracked fraction.

We claim as our invention:

1. Process for producing olefins suitable for conversion to detergent alkylate, comprising the steps of catalytically cracking a petroleum oil including a substantial proportion of components boiling above 300 C., fractionating the cracked products, recovering a fraction boiling above 300 C., selectively removing aromatic compounds solely from said fraction to produce an oil boiling above 350 C. and containing no more than 5% by weight of aromatics, thermally cracking solely said latter oil in vapor phase to produce a product comprising said olefins and recovering therefrom, by distillation, at least one fraction containing a high concentration of olefins having carbon numbers in the range from 8 to 18 and boiling in the range up to 350 C.

2. Process according to claim 1 in which said aromatics are removed by solvent extraction.

3. Process according to claim 2 in which said solvent is furfural.

4. Process according to claim 2 in which said solvent is aqueous phenol.

5. Process according to claim 1 in which said aromatics are removed by selective adsorption on a solid adsorbent.

6. Process for producing a detergent alkylate which comprises alkylating an aromatic hydrocarbon in the presence of analkylation catalyst with a fraction of olefins produced according to claim 1, having at least ten and no more than sixteen carbon atoms per molecule.

7. Process according to claim 6 in which said aromatic hyrocarbon is benzene.

8. Process according to claim 6 in which said catalyst is HP.

9. Process according to claim 6 in which said fraction of olefins has a carbon number spread of no more than 4 carbon atoms per molecule.

10. Process for producing a surface-active alkyl aryl sulfonate which comprises sulfonating a detergent alkylate produced according to claim 6.

11. Process for producing surface-active sulfuric acid alkyl ester salts which comprises reacting a fraction of olefins having at least 8 and no more than 18 carbon atoms per molecule, produced according to claim 1, with concentrated sulfuric acid to produce the alkyl sulfate and further reacting said sulfate with sodium hydroxide to produce the sodium alkyl sulfate.

(References on following page) References Cited in the file of this patent UNITED STATES PATENTS Frevel et a1. Feb. 13, 1951 10 Morris et a1 July 22, 1952 Good 'et a1 Sept. 23, 1952 Garner et a1 June 16, 1953 Mammen Sept. 20, 1955 Rappen et a1 Apr. 3, 1956 Keulemans et a1 Feb. 18, 1958 

1. PROCESS FOR PRODUCING OLEFINS SUITABLE FOR CONVERSION TO DETERGENT ALKYLATE, COMPRISING THE STEPS OF CATALYTICALLY CRACKING A PETROLEUM OIL INCLUDING A SUBSTANTIAL PROPORTION OF COMPONENTS BOILING ABOVE 300*C. FRACTIONATING THE CRACKED PRODUCTS, RECOVERING A FRACTION BOILING ABOVE 300*C., SELECTIVELY REMOVING AROMATIC COMPOUNDS SOLELY FROM SAID FRACTION TO PRODUCE AN OIL BOILING ABOVE 350*C. AND CONTAINING NO MORE THAN 5% BY WEIGHT OF AROMATICS THERMALLY CRACKING SOLELY SAID LATTER OIL IN VAPOR PHASE TO PRODUCE A PRODUCT COMPRISING SAID OLEFINS AND RECOVERING THEREFROM, BY DISTILLATION, AT LEAST ONE FRACTION CONTAINING A HIGH CONCENTRATION OF OLEFINS HAVING CARBON NUMBERS IN THE RANGE FROM 8 TO 18 AND BOILING IN THE RANGE UP TO 350*C. 